Method for connection acceptance control and rapid determination of optimal multi-media content delivery over networks

ABSTRACT

A method of bandwidth allocation for delivery of stored digital content from at least one server device to at least one client device by way of a network. The method includes the steps of: describing a control variable which represents a target flow rate of content from the server device to each client device; determining time-varying constraints on the target flow rate of the content; determining a cost function of the control variables for all clients, wherein the cost function represents an aggregate flow rate and is a sum of all flow rates for all clients; and prescribing bandwidth to all clients based upon a value of the control variables that maximize the cost function comprising performing periodic computations to update the value of the control variable such that the bandwidth can be continuously allocated to each client.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.11/031,416, filed Jan. 7, 2005, which is a Continuation of U.S. patentapplication Ser. No. 09/893,364, filed Jun. 26, 2001, (now U.S. Pat. No.6,850,965) which is a Continuation-in-Part of U.S. patent applicationSer. No. 09/344,688, filed Jun. 25, 1999, which claims priority U.S.Provisional Patent Application No. 60/108,777, filed Nov. 17, 1998, allof which are incorporated herein by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of delivery of multimedia contentover a variety of networks. More specifically, it pertains to multimediaservers which service many clients simultaneously for the delivery ofmultimedia content which is used and played back at each client. Itaddresses methods for determining optimal delivery rates to each clientand methods for determining whether new clients may be accepted withoutdiminishing the quality of service to existing clients.

2. Status of the Prior Art

In the history of multimedia program delivery, some in the industry havelong advocated the use of large client-side buffers andfaster-than-real-time content delivery over a network as offering thebest of all worlds: a jitter-free viewing experience and acost-effective utilization of the network resources at hand. Fewsystems, however, go very far in addressing how to schedule clients or amethod for accepting new clients. Real-time systems, often known asstreaming systems, can schedule new clients in a very simple manner—ifsufficient bandwidth remains for the added real-time stream, then theclient may be accepted. However, such systems do not maximize the numberof simultaneous clients. On the other hand, faster than real-timedelivery, sometimes known as store-and-forward systems, opens up thepossibility for more flexible scheduling procedures to control andoptimize the number of simultaneous clients while ensuring a high levelof quality of service.

The methods for such call acceptance and flow modulation that have beenproposed in the prior art have been largely ad-hoc and also incomplete.These have been ad-hoc in the sense that there has been no guidingrationale for their selection from among many possible and potentiallysuperior alternatives. The methods have also been incomplete insofar asthey did not address the question of whether any given incoming requestfor service should be accepted or denied. Video-on-demand systems, ormore generally, any system in which a multimedia server is designed toserve multiple clients over a network to deliver bounded content, canbenefit from the use of such flow modulation techniques and callacceptance procedures.

Optimal Content Flow Modulation

One time-honored way of designing methods of the class required here isto re-cast the problem to be solved as an optimization problem, in whichone seeks to maximize a designated value function moment-by-moment,subject to a set of real-world operational constraints which willtypically vary over time. Accordingly, given a set of clients andassociated sessions, an optimal delivery procedure continuouslyestablishes content flow rates from the content server to each of itsclients so as to maximize aggregate value according to the governingvalue function.

This approach holds several advantages: 1) optimization problems arewell understood, and are tractable by a large and diverse collection ofcomputational methods; 2) if it exists, a global solution that isobtained is arguably optimal by construction, and thus superior or equalto all other.

The present invention teaches the method of optimizing two particularvalue functions: 1) total data delivered (maximize throughput). 2) totaldelivery charges (maximize charges).

The first value function does not distinguish one customer from anotherand will deliver as much data as possible from server to clientsirrespective of the characteristics of the latter. The second valuefunction favors the service of high paying customers. It can easily beseen that the first function is a special case of the second one wherebyall clients are charged equally.

As will be seen in this disclosure, optimizing for these functions andidentifying the necessary constraints requires a new and uniqueperspective that is specifically designed for the multimediaenvironment. Moreover, the disclosed methods are specifically designedto account for and accommodate real-world scenarios of today's networks.Consequently many variations of the method are presented to accommodatevarious scenarios.

SUMMARY OF THE INVENTION

Call/Connection Acceptance Control (CAC)

A CAC procedure is responsible for deciding whether a candidate forservice can be accommodated without jeopardizing sessions already inprogress at the present time or at some time in the future; failing thatit must decide whether a service request should be queued for a time orrejected.

Flow Modulation

Flow modulation methods are those portions of the system which managethe communication and data flow between the server and the clients.Collectively, these methods provide the multimedia data to the clientand provide the server with the information about the state of thetransmission, playback, user status and network status. These parametersare further used by the present invention in the CAC procedures. Infact, as will be shown, the proposed CAC procedures are tightlyintegrated with the flow modulation methods.

Adaptation to Variations in Network Capacity

Operational constraints may change over time. For instance, one mightelect to vary the total bandwidth available for multimedia contentdelivery according to the time of day. Alternatively, exogenous dataflows on the network may cause unexpected disturbances by usurpingavailable bandwidth and impeding the delivery of data along establishedsession channels. The content delivery strategy of the present inventionincludes the ability to adapt to scheduled as well as unexpecteddisturbances so as to minimize unwanted disruptions of services.

Burst Transmissions Provide the Opportunity to Adapt

The present invention, due to it's faster-than-realtime transmissions(also know as burst transmissions), which are realized by use ofhigh-bandwidth networks and large client cache or intermediate storage,provides an opportunity to adapt to changing network conditions. Incontrast real-time (streaming) systems are essentially designed forworst-case scenarios: each client must be assumed to constantly use thecomplete real-time playback bandwidth. Such a system is unable to adaptto any derivation from this scenario. For example, take the simple casewhere the total server bandwidth is 100% utilized by all clients playingback the streaming video. Should any network condition change, such as atemporary decrease in available bandwidth over the network, then one ormore clients' playback is interrupted, and the system can not recoverfrom such a condition until the bandwidth is regained. Even worse if asingle client presses pause either that unused bandwidth must remainreserved and no more clients can be accepted, or that paused client ispushed out in order to service the new client. In essence little or noCAC procedure may be implemented.

In contrast the present invention burst transmits portions of a programand immediately gets ahead of itself, thus allowing headroom for amyriad of methods to intelligently handle new clients, clientinteractivity and possible network fluctuations.

Methods are taught for optimally determining the flow rate to eachclient. Methods are also taught for accepting or rejecting new clients;these call-acceptance methods are tightly coupled with said flow ratemodulation methods. A series of constraint expressions are presentedwhich govern the methods for determining the flow rates and acceptanceof new clients. Linear programming techniques are used to optimallysolve these expressions. Various embodiments are presented includingscenarios for multiple-rate tariffs, and time-of-day bandwidthvariations.

BRIEF DESCRIPTION OF THE DRAWINGS

These as well as other features of the present invention will becomemore apparent upon reference to the drawings wherein:

FIG. 1 depicts the flow of control and/or data between the differentstations of a content delivery session;

FIG. 2 illustrates the Entity Data Model;

FIG. 3 geometrically illustrates the linear programming problemstatement;

FIG. 4 a geometrically illustrates an expansion of the flow optimizationproblem statement;

FIG. 4 b geometrically illustrates the method for rapid determination offlow rates to maximize flow;

FIG. 5 illustrates a method for implementing flow modulation to maximizeflow;

FIG. 6 illustrates a method for implementing flow modulation formaximized charges;

FIG. 7 illustrates typical content flow;

FIG. 8 illustrates typical server swing capacity;

FIG. 9 illustrates a method for call-acceptance and control (CAC);

FIG. 10 illustrates planned constraints on maximum flow;

FIG. 11 illustrates a method for call-acceptance and control (CAC) withscheduled flow changes;

FIG. 12 illustrates stratification of services;

FIG. 13 illustrates a method for call-acceptance and control (CAC) formaximal charge;

FIG. 14 is a system block diagram for a system implementing the presentoptimization of client bandwidth;

FIG. 15 is a flowchart showing an algebraic method for calculating theoptimization of client bandwidth;

FIG. 16 is a flowchart showing an algebraic method for calculating theoptimization of client costs; and

FIG. 17 illustrates optimized viewing of a live event.

DETAILED DESCRIPTION OF THE INVENTION

Data & Control Flows (FIG. 1)

FIG. 1 depicts the flow of control and/or data between the differentstations of a content delivery session. As shown a client attempts aconnection 100 and manifests itself to the Content Selection subsystemby means of a low bandwidth control channel (not shown). Next the clientis authenticated and a selection is made 110, typically with the aid ofbrowser software. If the client is not authenticated, it is dismissedfrom the system 120. If the client has been authenticated and a programselected for viewing then the rate of service is set at this point 130,perhaps according to the selection that was made, or some contractualstipulation. The client is now placed on the service queue of the CACsubsystem 140. A client that is made to wait too long will eventuallybalk 150. Assuming this does not occur, the CAC subsystem 140 willeventually allocate a channel to the client and open a session 160.Control now devolves upon the Content Flow Modulator (not shown) whichstarts the flow of content from server to client 170. Subsequentcapacity changes, whether predictable or not, may force an abrupttermination of a session in progress 180. Otherwise the session runs tocompletion 190.

Entity Data Model (FIG. 2, Listing, Table)

The entities entering into our discussion are depicted in FIG. 2. Client200 maintains certain data associated with this entity; as shown but notlabeled, which includes without limitation, status, id andcostOfService. The other entities also each include unlabeled butdepicted data. The diagram further depicts the relationship between eachentity. As shown, client 200 is assigned a session 240. Client 200employs a channel 210. Client 200 selects contentSelection 230. Session240 delivers content through channel 210. Server 220 modulates channel210. Server 200 contains contentSelection 210. Server 220 accepts,defers or denies client 200 and contentSelection 230 is associated withsession 240.

Furthermore FIG. 2 depicts the various one-to-many relationships. Eachclient 200 employs one channel 210. Client 200 may or may not receiveone of channel 210, as depicted by the 0/1 notation. Similarly, client200 may or may not receive a session 240. However, whenever client 200does receive a session 240, it will always receive a channel 210 sincechannel 210 and session 240 are allocated as a pair. One or more (N) ofclient 200 may select one of contentSelection 230. And server 220contains one or more (N) of contentSelection 230. Each one ofcontentSelection 230 is associated with 0-N of session 240. Each session240 delivers content through one of channel 210. And server 220modulates one or more (N) of channel 210.

A more detailed list of each entity of FIG. 2, and each one's associateddescription, data elements and function calls is listed below. Thislisting closely resembles that of object-oriented programming. As such,‘methods’ represent the ability to obtain or modify data, while‘attributes’ represent data which is directly associated with thatparticular entity. The listing also includes information relating to oneembodiment wherein software programming specifics are disclosed, such asa variable type (double, int and so forth) and more. The presentinvention is not limited to such an embodiment and other implementationsare possible without deviating from the scope and intent of the presentinvention. The listing, however detailed, is merely illustrative of thedata and functions which are used in the equations and methods describedherein.

Consequently, data and functions from this listing, associated with thevarious entities, will be used in forthcoming equations, flowcharts andmethods. The reader is directed to this listing as reference whenreading such equations and examining such drawings.

—Start of Entity Data Model Detailed Listing—

Model: Untitled 1 (Public)

Contains:

client, session, channel, server, contentSelection.

Component: Client (Public Class/Interface)

Comment:

A client entity stands for a client presently requesting or receivingservice.

Methods:

public static lookup (id: in int): client

public GetId ( ): const int&

public SetId (val: in int&)

public GetCostOfService ( ): const double&

public SetCostOfService (val: in double&)

Attributes:

private status: client<int>

Specifies whether or not a client has been allocated a channel andsession.

private id: int

Integer-valued identifier that is unique to the client (primary key).Can be obtained from a monotonically increasing counter.

private costOfService: double

Dollar charge per Mbyte. This value is the same for all customers underflow optimization. Under cost/charge optimization may be an integervalue reflective of the rank; the higher the rank the higher the charge.

Has:

public selected: contentSelection

public assigned a: session

public employs: channel

Component: session (public Class/Interface)

Comment:

A session entity holds various state information about the service beingreceived by an associated customer.

public GetCurrentPosition ( ): const double&

public SetCurrentPosition (val: in double&)

public GetPayloadToGo ( ): const double&

public SetPayloadToGo (val: in double&)

public GetStatus ( ): const int&

public SetStatus (val: in int&)

public GetMinimumFlowRate ( ): const double&

public SetMinimumFlowRate (val: in double&)

public GetFlowRateRange ( ): const double&

public SetFlowRateRange (val: in double&)

public GetMaxFlowRate ( ): const double&

public SetMaxFlowRate (val: in double&)

Attributes:

private playTimeToGo: double

Indicates the minutes remaining in the viewing experience. Initializedto contentSelectiont.playTime (see below).

private currentPosition: double

Pointer into media content from which content is being delivered.

private payloadToGo: double

The amount of media content (in Mbytes) as yet undelivered by theserver. Does not include any content presently stored in the client-sidebuffer.

private status: int

Indicates whether session is active or paused.

private minimumFlowRate: double

This is the minimum flow from server to client required to ensureuninterrupted service over the remaining playTime. Has a value of zeroif playloadToGo is zero. Given by (payloadToGo*8)/(playTimeToGo*60)

private flowRateRange: double

Specifies the effective range over which the channel content flowserving a session is constrained without consideration for interactionswith other flows.

Equals maxFlowRate−minimumFlowRate

private maxFlowRate: double

Effective maximum bound on flow as expressed in formula (8) which mustbe re-evaluated periodically.

Has:

public delivers content through: channel

Component: channel (public Class/Interface)

Comment:

A channel represents the network resources from server to clientassociated with an ongoing session, encompassing the client-side bufferif any, and its level.

public GetBufferLevel ( ): const double&

public SetBufferLevel (val: in double&)

public GetFlowRate ( ): const double&

public SetFlowRate (val: in double&)

public GetMaxFlowRate ( ): const double&

public SetMaxFlowRate (val: in double&)

Attributes:

private bufferSize: double

Capacity of the client-side buffer (or equivalent),

private bufferLevel: double

Current buffer level in MBytes of stored content.

private flowRate: double

Flow rate through channel specified by the relevant optimizing flowmodulator.

private maxFlowRate: double

This value represents the maximum possible flow rate from the server toan individual client over its “channel”. This value reflectsrestrictions on flow that pertain to an individual client. It may bedetermined by factors such as the bandwidth of client's link to thenetwork, or a limit imposed administratively to ensure balanced networkutilization.

Component: server (public Class/Interface)

Comment:

Entity representing the media server and its CAC and flow modulationactivities.

public GetFlowRate ( ): const double&

public SetFlowRate (val: in double&)

public GetMaxMinFlowRate[ ]( ): const double&

public SetMaxMinFlowRate[ ](val: in double&)

Attributes:

private maxFlowRate: double

Maximum possible content flow that is allocated to the server by thenetwork.

private FlowRate: double

Aggregate content flow rate, summed over all sessions and theirassociated channels.

private cac_flowSafetyMargin: double

Tunable safety margin used by the CAC algorithm to protectsessions-in-progress from being affected by changes in available networkbandwidth.

private maxMinFlowRate[ ]: double

Applies when N rate tariffs exist. This array holds the maximum floorlevel for each category of service. The value for the costliest categoryN is stored in maxMinFlowRate[N−1], and for the least costliest inmaxMinFlowRate[0]. It is the relative magnitude of these ascendingvalues that matters, not their absolute value. Thus the actual maximumfloor flow rate for category k is given byserver.maxFlowRate*(server.maxMinFlowRatc[k−1]

/server.maxMinFlowRate[N−1]). Similarly, the maximum floor flow rate forcategory N is server.maxFlowRate.

Has:

public contains: contentselection

public modulates: channel

Component: contentSelection (public Class/Interface)

Comment:

Entity represents a video/sound clip or other bounded unit of content. Acontinuous data feed does not qualify.

Attributes:

private averagePlayRate: double

The average rate at which media content is consumed by the client, ascomputed by dividing the (payload*8) by the (playTime*60)

private playTime: double

Duration of play of the media content in minutes.

private payLoad: double

total size of the content in Mbytes.

Has:

public is associated with: session

—End of Entity Data Model Detailed Listing—

The following table summarizes the highlights of the previous detaileddescription of each entity in FIG. 2. TABLE 1 Entity Description client200 Each client is denoted by an associated unique integer index_(Id).The set of active clients is denoted by S_(activeClients). The set ofdeferred clients is denoted by S_(QdClients). Incoming clients areexpected to select the content they wish to view prior to being queuedfor dispatch by the CAC sub-system, which requires knowledge of theclient's bandwidth requirements, duration of play, and cost of service,all of which may vary according to the selection. server 220 Servers sitastride a network and can deliver media content through the network totheir clients up to a designated maximum flow rate. The server isresponsible for accepting or rejecting clients, launching sessions andassociated channels for the former, and modulates content flows over allchannels in an optimal manner. channel 210 A channel represents the datapath between the server and the client. The channel buffer is typicallylocated near or within the client's viewing station. The flow of contentthrough the channel is set by the flow modulator sub-system.contentSelection A server will typically act as a repository for mediacontent, 230 which it can deliver to clients upon demand. For purposesof the invention, media content is characterized by its payload and theplay duration, which together imply the averagePlayRate = (payload * 8)/(playTime * 60). The averagePlayRate is the streaming rate imposed byrealtime just-in-time streaming algorithms. session 240 Every sessionrepresents an instance of media content delivery to an associated clientover a designated channel. The playTimeToGo indicates the time remainingbefore the content is fully played out to the client. The payloadToGo isthe amount of content data as yet undelivered to the channel. A sessionterminates when this value reaches zero, at which time playTimeToGo maystill be large, according to the capacity, the level of the channelbuffer, and the media play rate.Constraints on Content Flow

Before referring to more Figures, it is imperative to establish someformulas and problem statements which are used in the methods whichfollow.

The flow of content between entities is subject to the followingconstraints at all times. Buffer levels are always expressed in Mbytesand data rates in Mbits/sec.Σ_(iεSactiveClients)(client.lookup(i).channel.flowRate)<=server.maxFlowRate  (1)

The sum of all channel flows cannot exceed the imposed maximumthroughput capacity of the server. (2) client.lookup(i).channel.flowRate<= client.lookup(i).channel.maxFlowRate for all i ε S_(activeClients)

The data path from server to client is subject to its own constriction.(3) client.lookup(i).channel.flowRate <=(client.lookup(i).channel.bufferSize −client.lookup(i).channel.bufferLevel) * 8/60 +client.lookup(i).session.mediaContent.averagePlayRate, for all i εS_(activeClients,)

The channel buffer is never allowed to overflow. (4)client.lookup(i).channel.flowRate <=client.lookup(i).session.payloadToGo * 8/60 for all i εS_(activeClients,)

Content that does not exist can not be delivered. (Constraint 1 willordinarily prevail except at the very end of a session.)

The constraints listed above are straightforward applications relatingto the flow of data through constricted channels, out of finite datasources, and into and out of bounded buffers. By contrast, the followingconstraint, which imposes a minimum channel flow rate instead of amaximum, is less obvious. The minimum value, termed the minFlowRate isset to the flow rate which, if sustained over the balance of the playtime to go (playTimeToGo), ensures that all required content will beavailable when needed—and no sooner—until all content is played out.This floor value can be calculated for i ε S_(activeClients) by theformula (5) client.lookup(i).session minFlowRate =(client.lookup(i).session.payloadToGo * 8)/(client.lookup(i).session.playTimeToGo * 60)

Accordingly, the flow rate on the right-hand-side is termed thejust-in-time (JIT) flow rate (f^(JIT)).

Thus: (6) client.lookup(i).channel.flowRate >=client.lookup(i).session.minFlowRate for all i ε S_(activeClients)

The variable constraint bounds (i.e. the values to the right of theinequality symbol) of equations 1-4 and 6 are re-evaluated on a periodicbasis (e.g. once per second) prior to the execution of the CAC procedureand optimizer. In particular, the minFlowRate value starts out at thebeginning of a session equal to the streaming rate. By construction theminFlowRate rate never exceeds this initial value so long as constraint6 is honored. In fact, constraint 5 implies that the minflowRate valuemust be a diminishing function of time that may hold its value for atime but never rises. As seen from equation 6, the actual data rate ofthe channel, flowRate, is always greater than or equal to theminFlowRate. By design, and virtue of the fact the present inventionuses faster-than-realtime transmissions, the system quickly gets aheadof itself and ensures that after initial conditions, the minFlowRate isalways equal to or less than the real-time rate and that it continues todecrease. As we shall see, the CAC procedure exploits this monotoniccharacteristic of the minimum flow rate over time.

Constraints 2, 3 and 4 are of like kind, each specifying an upper boundon individual channel flows. Whereas the bound for constraint 2 istypically a constant, the bounds on 3 and 4 will vary over time.Nevertheless, only one of the three bounds is effective at any giventime, namely the one with the smallest bound value, given by: (7)client.lookup(i).session.maxFlowRate = minimum of 1)client.lookup(i).channel.maxFlowRate, 2)(client.lookup(i).channel.bufferSize −client.lookup(i).channel.bufferLevel ) * 8/60 +client.lookup(i).session.mediaContent.averageplayRate, 3)client.lookup(i).session.payloadToGo * 8/60

Consequently, formulas 2, 3, and 4 can be consolidated into a singleconstraint, the bound for which is computed at every scan to be thesmallest bound of associated constraints 2, 3 and 4.client.lookup(i).channel.flowRate<=client.lookup(i).session.maxFlowRate,  (8)whereby for all i ε S_(activeClients), maxflowRate is given by equation(7).

At any one time, individual channel flows are constrained over a range,as follows: (9) client.lookup(i).session.flowRateRange =client.lookup(i).session.maxFlowRate −client.lookup(i).session.minimumFlowRateValue Functions

The value functions introduced in the Description of Related Art can nowbe expressed mathematically as linear functions of channel flows asfollows: Optimizing Throughput (Maximal Flow) (10) value =Σ_(i ε SactiveClients) client.lookup(i).channel.flowRate) OptimizingCharges (Maximal Charges) (11) value = Σ_(i ε SactiveClients)(client.lookup(i).channel.flowRate) * client.lookup(i).costOfService)Optimization Problem Statement (FIG. 3)

The optimization problem, which in one embodiment is strictly throughputand in another case is charge, can be stated simply as follows:

Find values for

client.lookup(i).channel.flowRate for all i ε S_(activeClients)constrained by inequalities 1 through 5, such that the value obtained byevaluating expression 10 or 11 assumes a maximum.

Both of these problem formulations are examples of Linear Programmingfor which a number of well-known and generally effective computationalsolutions exist. In linear programming one seeks to optimize a linearcost function of variable xc*x=c ₁ *x ₁ + . . . +c _(n) *x _(n)   (12)subject to a set of linear inequality constraintsA*x<=bwhere x^(T)=(x₁, x_(n)), c=(c₁, . . . , c_(n)) are the state variable &cost vectors, A is an n-by-m matrix, b^(T)=(b_(i), . . . b_(m)) is theconstraint vector, and the operator ‘*’ stands for matrix or scalarmultiplication.

FIG. 3 is introduced as illustrative of the problem statement and thegeneral methods of the prior art, and is not incorporated as an elementof the invention.

The linear programming problem as well as its solution can best beunderstood with the aid of geometry. FIG. 3, depicting a 2-dimensionalCartesian problem space, inequality constraints (13) define a convexhull H 310 over which a search for an optimum value of x=(x₁,x₂) ispermitted to range. The cost vector c350 defines an infinite family ofequal cost lines (hyperplanes) which lie orthogonal to c. Three examplesof such lines are shown in L₁ 360, L₂ 365, and L₃ 370., each ofprogressively higher value. The supreme value of the cost function isobtained by sliding along c 350 till one can go no farther, in thisinstance toward vertex V₄ 340 of hull H 310. Many well-known methods(e.g. the Simplex Method) work roughly in this fashion, exploiting thefact that at least one optimum point must be at a vertex. In particular,the Simplex method algorithm begins by finding a vertex (e.g. V₂ 320),and then moves along a sequence of vertices (e.g. V₃ 330, V₄ 340)improving itself each time until no further improvement is possible &the summit is reached.

Let us suppose instead that V₃ 330 were placed along L₃ 370 along withV₄ 340. According to prior art methods, V₃ 330 and V₄ 340 are the twopossible solutions, but the equally valuable points in between them arenot. As we shall soon see, the problem of throughput optimization (6)falls in this category.

While vertex V₁ 300 does not factor into this description, it isdepicted in FIG. 3 for completeness.

Flow Modulation

Methods for Maximal Flow

The following sections detail two embodiments to optimize total dataflow.

Overview (FIG. 4-a)

FIG. 4-a depicts a scenario involving two flows. The convex hull is inthis instance bounded by line segments L1, L2, L3, L4 and L5. L6 is aboundary used in a different embodiment, however the present embodimentuses L5 and not L6. Flow f₂ can range over the interval separating linesegments L1 from L3, namely f₂ ^(MIN) and f₂ ^(MAX); the range isdepicted as f₂ ^(RANGE). Flow f₁ can range over the interval betweenlines L2 and L4, namely f₁ ^(MIN) and f₁ ^(MAX), and depicted as f₁^(RANGE). The sum of flows f₁ and f₂ is constrained to lie inside ofline segment L5 which, by construction, is always orthogonal to the costvector C_(f). Cost vector C_(c) is also illustrated but used in adistinct embodiment. In the present embodiment only C_(f) is used. Inthe illustrated example of the present embodiment, the constraint ontotal flow is set to 5, and is therefore low enough to cause L5 tointersect L3 and L4. This would not have been true had the value chosenhad been 10 instead of 5. With L5 safely out of contention, the convexhull would instead be a simple rectangle bounded by L1 through L4,thereby permitting both flows to assume their respective maxima withoutinterference. In practice operational constraints exist intrinsically orare imposed from the outside so as to ensure cost effective sharing ofpotentially costly network resources.

Supposing FIG. 4 to be correct, the well-known methods would selectvertex V₃₋₅, which lies at the intersection of L3 and L5, or V₄₋₅, whichlies at the intersection of L4 and L5. These solutions, though optimal,are undesirable for the present invention as they fail to spreadavailable bandwidth over all channels as fairly as would a centrallylocated interior point of L5. For this reason, two alternativeoptimization methods are taught, which are adapted to the particularneeds of this problem and ensure a fairer allocation of constrainedbandwidth among all channels.

Iterative Procedure (FIG. 5)

In order to optimize use of all available bandwidth, the followinggeneral method is used, with the details illustrated in FIG. 5. Thismethod is a solution for the problem illustrated in FIG. 4-a, whichgeometrically illustrates the optimization problem in the limited caseof two flows, f1 and f2. The following description expands the problemto an arbitrary number of clients (and therefore flows) and presents amethod for solving this optimization problem.

Referring to FIG. 5, in step 500, values are calculated for the sessionmaxFlowRate and session.minFlowRate for each client as per the minimumand maximum constraint bound expressions in equations 6 and 8,respectively. This step correlates to the determination of f₁min, f₁max,f₂min and f₂max from FIG. 4-a.

The difference between these two yields the session.flowRateRange ofeach client. Thus:session.flowRateRange=session maxFlowRate−session.minimumFlowRate

In step 505, the active clients are sorted in an ascending fashion basedupon their session.flowRateRange. As will be shown this critical stepfacilitates allocation of the remaining server bandwidth as evenly aspossible among all active channels, thus maximizing the number ofchannels that benefit by use of the total server bandwidth. An arbitraryassignment of remaining bandwidth is likely to saturate the serverbefore all channels have been assigned extra bandwidth, thereby favoringcertain channels on an ad-hoc basis. This step correlates to keeping thesolution in bounds of the space delinated by line segments L1-L5 of FIG.4-a.

In step 510, each client's channel flow rate is set to the sessionmimimumFlowRate.

By doing so, it is ensured that the minimum flow constraint is met foreach session and that the minimum flow rate is a non-increasing functionof time, which is critical to the proper functioning of the CACprocedure. This portion of the process also ensures that the solution,starting from the vertex in FIG. 4-a as defined by the intersection ofL1 and L2, moves generally in the direction of vector c_(f). All clientsare also marked as unprocessed.

In the next step, 520, server.flowRate is set to the sum of each activeclient's session.flowRate.

Next, the following is iterated over all clients in sorted sequence(during any given iteration the selected client is given by its id) byperforming steps 530 through 570. In step 530 evaluating the followingexpressions test for possible server saturation: delta =(server.maxFlowRate − server.flowRate)/ (qty of un-processed clients)range = client.lookup(id).session.maxFlowRate −client.lookup(id).session.flowRate

If range is greater then delta, this implies that the server can besaturated in this iteration by allocating delta to all unprocessedclients (step 540).

On the other hand, the ‘no’ path for step 530 implies that the server isnot saturated and that the present client (given by id) will saturatefirst. Accordingly, in 550 the delta variable is set as follows:delta=range

To again correlate this process back to the geometry of FIG. 4-a, serversaturation is indicated when the solution which is being sought in thedirection of vector c_(f) goes beyond line segment L5.

Next, the flow rate is incremented for all unprocessed clients by delta,causing client id to saturate.

In step 560 the server flow rate is adjusted accordingly:server.flowRate=server.flowRate+delta*(qty of unprocessed clients)

In step 570 the client given by id, now saturated, is marked asprocessed

Algebraic Procedure (FIGS. 4-b and 15) By Interpolation

Referring to FIG. 4-b, the iterative method described in the previoussection begins its search for a maximum from vertex V_(min) along thedirection shown by line C₁. In contrast, the present method follows thediagonal line segment L_(diag) of the rectangle bounded by lines L1through L4, starting at vertex V_(min) and ending at point V₃₋₄.

Provided it exists, the intersection between L_(diag) and L5, indicatedby V_(f), is optimal on the same basis as any other point lying alongthe intersection of L5 with the rectangle bounded by L1 through L4, aspreviously discussed.

Whenever L5 does not intersect this rectangle, the optimal solution isgiven by vertex V3-4 at which f1 and f2 both assume their respectivemaxima.

In the first instance, the coordinates for point V_(f) can be obtainedby elementary vector geometry and algebraic manipulation, as follows:

-   -   First, we define a number of abbreviations,    -   f_(k) ^(min) represents client.lookup(k).session.minimum flow        rate    -   f_(k) ^(range) represents client.lookup(k).session.flow rate        range    -   f_(svf) ^(max) represents server.maxflowrate

We seek scale factor x such that the vector sum of (f₁ ^(min) . . .f_(n) ^(min))+x(f₂ ^(range) . . . f_(n) ^(range)) intersects anequi-cost hyperplace for capacity c.

The point of intersection is obtained by solving equation:(f ₁ ^(min) + . . . +f _(n) ^(min))+x(f ₁ ^(range) +. . . +f _(n)^(range))=cSolving for x, we obtain:x=[c−(f ₁ ^(min) + . . . +f _(n) ^(min))]/(f ₁ ^(range) + . . . +f _(n)^(range))

where f_(svr) ^(max)−(f₁ ^(min) + . . . f _(n) ^(min)) represents theunused bandwidth beyond the minimum allocation for each session. For anygiven session, the optimal flow rate is given byf _(i) =f _(i) ^(min) +x f _(i) ^(range), if x<1,orf _(i) =f _(i) ^(min) +f _(i) ^(range,) if x>=1

This solution by interpolation is interesting by virtue of the greatefficiency of the calculation, and the fact that no session saturates(x>=1) unless all sessions saturate. In contrast, the previous methodentails considerably more computational effort, and tends to saturatesessions with the lowest reserve capacity.

A block diagram for this algorithm is depicted in FIG. 15. Block 1500establishes the maxim and minima for each flow. For each flow theminimum is subtracted from the maximum to obtain the range. AvailableBandwidth is calculated in block 1520, as the difference between theaggregate flow capacity and the sum of flow minima. Factor x iscalculated next in block 1540, and tested against 1 in block 1650. Flowrates for each session are computed in block 1560 if x is less than 1,or 1570 in the contrary case.

A Method for Maximal Charge

The following sections detail one embodiment to optimize the totalmonetary charges within the system. The second method is algebraic innature, and thus very efficient.

Overview (FIG. 4-a)

Referring back to FIG. 4-a, cost vector C_(c) lies orthogonal to lineL6, which intersects the convex hull at the vertex formed by theintersection of lines L4 and L5, namely V₄₋₅. This cost vector, and theoptimal point that it implies, favors flow f1 over flow f2. In thisexample, as the cost of service for f1 equals 2, thus exceeding the costof service of 1 set for f2. As the number of flows grow to exceed thenumber of distinct categories of service (and associated costs ofservice) the unique optimal solution, depicted in FIG. 4 for the casewhere every flow has a distinct cost of service, no longer applies. Onceagain a plurality of flows within a service category vie for bandwidthwhich a method should endeavor to distribute evenly. This method isderived from the previous one, and optimizes one cost category afteranother, starting with the most costly and ending with the least costly,or when all available capacity is allocated.

An Iterative Search Procedure (FIG. 6)

Let the service categories be denoted by k=1 . . . N, where k alsodenotes the cost of service.

Let C₁ . . . C_(N) be the partition Of S_(activeClients) that places allclients with cost of service k in set C_(k). Partition sets C_(k) can beordered to form sequence SeqC=C_(N) . . . C₁.

FIG. 6 depicts the method for implementing the method to maximize thecost of service (service charge).

This method is nearly identical to the iterative procedure used tomaximize flow. The principle difference stems from the partitioning ofclients according to their category (cost) of service: clients chargedmost are allocated bandwidth preferentially. This is accomplished byadding another level of iteration around the method of FIG. 5. The inneriteration (steps 650 through 680) functions exactly as before, with thedifference that its actions are limited to the clients belonging to thegiven service category k (i.e. C_(k)). This difference also holds trueof step 640 which sorts category k clients according to their flowranges prior to entry in the bandwidth-allocating inner loop. The outerloop proceeds down a sorted sequence of service categories SeqC(generated in step 630), starting with the category generating thegreatest revenue to the service provider. Given a fairly static set ofservice categories, this sort need be performed only when the categoriesundergo change. Steps 670, 675 and 680 are identical to theircounterparts in the method of FIG. 5 (i.e. 570, 575 and 580).

The net effect of this method is preferential allocation of bandwidthaccording to category of service, and equitable treatment of clientswithin the same category of service.

An Iterative Algebraic Procedure (FIG. 16)

The algebraic method used to maximize bandwidth (FIGS. 4 b and 15) canbe viewed as a special case of cost optimization in which all costs areequal. By construction, all sessions belonging to a given category,starting with the most expensive category, and ending with the least,and updating available bandwidth after each step in sequence, we obtainan iterative algebraic method that scales much better than the precedingmethod.

A block diagram for this algorithm is depicted in FIG. 16. Block 1600establishes the maxima and minima for each flow. For each flow, theminimum is subtracted from the maximum to obtain the range. In block1610, every session flow is assigned the minimum allowed value, whichvalue will apply by default should the iterative procedure run out ofbandwidth before all categories can be considered. Block 1620 computesthe initial value for available bandwidth, as the difference between theaggregate flow capacity and the sum of flow minima.

Flow values are obtained for each cost category in order of decreasingcost within blocks 1640 through 1675. Factor x is calculated next inblock 1540, and tested against 1 in block 1650. Flow rates for eachsessions of the category under consideration are computed in block 1660if x is less than 1, or 1670 in the contrary case. In the former casethere is no more bandwidth to allocated to the sessions in the categoryunder consideration.

Call Acceptance Control (CAC)

CAC for Maximal Flow Overview (FIGS. 7-8)

The CAC procedure applicable to this flow optimization relies on theessential step of accepting a new client if and only if the added loadinduced thereby does not compromise service to existing clients or thenew one. This critical step could not be accomplished without the closeintegration with previously-described flow-modulation methods of FIGS.5, 6, 15 and 16.

According to the previous discussion, the minimum flow rate is theminimum sustained flow rate that guarantees that the associated viewerwill not be subject to interruptions in service due to a shortfall ofcontent from the media server. It follows that whenever data is beingdelivered at a rate in excess of the minimum flow rate, a downwardadjustment toward the minimum level could be accommodated as needed tosurrender bandwidth to any newcomer.

FIG. 7 depicts content flow over a channel for the course of a typicalsession, and also how data is delivered under real-time streaming D. Theamount of content delivered is the same in either case, but the mannerof delivery differs considerably. A session is launched at time 0 as thenetwork is lightly loaded, and the optimizer sets an accordingly highflow rate. Another client emerges at the end of interval 700, causing adownward adjustment to the flow rate over interval B, as availablebandwidth is shared between two sessions. During both of these intervalsthe minimum flow rate 720 drops quickly, as data accumulates in theclient's media buffers. At the end of interval B, a massive influx ofclients necessitates that flow be dropped to the minimum flow rate,which now lies substantially below the streaming rate D and is helduntil all data is delivered at the end of interval C. Note that theminimum flow rate, shown as element 720, diminishes monotonically overtime.

The server swing capacity is defined as the difference between themaximum capacity of the server and the total minimum flow rates for allactive clients. Therefore: (14) swingCapacity = server.maxFlowRate −Σ_(iεSactiveClients)(client. lookup(i).session.minFlowRate)

Given the monotonic decreasing nature of session minimum flow rates,server swing capacity can readily be seen to be a monotonic increasingfunction of time over the intervals separating client admissions, atwhich point it undergoes a drop as a new load is taken on. Thisall-important characteristic implies the following:

Any client admitted for service based on the present value of swingcapacity is guaranteed to have sufficient bandwidth at its disposal overthe entire future course of the session.

FIG. 8 depicts the server swing capacity 800 over the course of thesessions illustrated in FIG. 7. Swing capacity rises quickly overintervals A & B as data is delivered at high flow rates over thenetwork. It holds steady over interval C when all channels flow at theirminimum rate then jumps at the end of C before resuming its monotonicrise once again.

Procedure (FIG. 9)

In this procedure, which executes on a periodic basis, queued clientsawaiting bandwidth are scanned in FIFO order. For each one, the requiredbandwidth is computed as per the client's prior content selection. Ifthe available swing capacity (reduced by a safety margin) exceeds theamount required, then the client is activated and swing capacity isadjusted accordingly. Otherwise, two possible cases are considered: 1)under the FirstFit embodiment, the procedure continues scanning clientsto the end of the queue, activating clients whose requirements can bemet; 2) under the FIFO embodiment, the procedure ends with the firstcandidate client whose requirements cannot be met.

In step 900, available server swing capacity is evaluated according tothe formula swingCapacity=server.maxFlowRate-Σ_(iεSactiveClients)(client.get(i).session.minimumFlowRate)

The bandwidth requirement for client id in Step 920 is obtained asfollows:required_bandwidth=client.lookup(id).contentSelection.averagePlayRate

The predicate evaluated in Step 940 is given by the expression:(required_bandwidth<=swingcapacity-server.cac_flowSafetyMargin)

In step 950, client activation entails allocation of a session and achannel, and insertion in the set of active clients eligible forbandwidth allocation by the optimal flow modulator.

In step 960 the swing capacity is diminished by the amount reserved forthe activated client:swingcapacity=swingCapacity-required_bandwidth;Responding to Variations in Network Capacity (Maximal Flow)

In the CAC procedure for maximal flow, a safety margin was introduced,namely server.cac_flowSafetyMargin, to provide the means for ensuringthat the server's swing capacity will never fall below a minimalthreshold value.

According to this procedure, the following inequality always holds true:swingcapacity>=server.cac_flowSafetyMargin

In the previous discussion, a server's swing capacity provided the basisfor determining whether or not a prospective client should be allocatedbandwidth.

By holding server.cac_flowSafetyMargin in reserve, the CAC algorithmforces delivery of content at faster than real-time rates among acceptedclients, even under the heaviest load. The net effect is to apply upwardpressure on client side buffer levels, thus promoting continuousaccumulation of content and a jitter-free viewing experience once asession is under way.

In another embodiment, server swing capacity can also be interpreted asspecifying the maximum amount by which the server.maxFlowRate constraintcan be dropped without affecting service, should such an adjustmentprove necessary due, for instance, to an influx of exogenous networktraffic that diminishes the amount available for multi-media services.Parameter server.cac_flowSafetyMargin can thus be set so as to guaranteea minimum capacity to tighten the constraint on maximum server flow inresponse to unexpected load changes that affect the server's ability toservice its existing clients as well as new ones.

Anticipating Scheduled Variations in Network Capacity (Maximal Flow)Overview (FIG. 10)

FIG. 10 depicts how the constraint on maximum flow might be allowed tovary according to the time of day, day of the week, and so forth, inexpectation of time-varying traffic flow levels extrapolated from pastexperience, traffic flow models, etc. Maximum flow rate 1000 can be seento vary based upon the time of day. In practice, defining theright-hand-side of inequality constraint 1 as a time-dependentexpression can impose such time-varying capacities. According to theprevious description, the optimizer, which executes on a periodic basis,will automatically seek new flow levels for every active session as theconstraint varies. There is, however, no guarantee that an acceptableoperating point will be found for all sessions (i.e. one that respectsthe minimal and maximum constraints on session channel flow). One suchexample is the case where the server is loaded to the limit and totalcapacity is curtailed in excess of the aforementioned safety margin.Should such a situation arise the only recourse may well be thetermination of a number of established sessions (i.e. load shedding).

The goal is to eliminate service disruptions of this sort by allowingthe CAC procedure to look ahead into the future, and accept new clientsonly if these can be accommodated without any compromise in service inthe midst of previously anticipated changes in available networkcapacity. The following CAC procedure generalizes the previous one:before accepting a client, the test on swing capacity is repeated over asequence of time segments that cover the proposed viewing period.

Definitions

Let:t_end(i)=client.lookup(i).session.playTimeToGo+t_now   (16)

Let server.maxFlowRate(t) be server flow capacity as a function of time,as exemplified in FIG. 10.

Let Seq_(T)(t_now)=advancing sequence of future times, lead by t_now,when server.maxFlowRate(t) undergoes a step change. For instance, at9:15 in FIG. 10 this sequence reads as follows: 9:15, 9:30, 11:30,13:30, 6:30, 7:30.

The server swing capacity at a future time t is computed according tothe capacity and worst-case client flows at time t. (17)swingCapacity(t)=server.maxFlowRate(t)−Σ_(t) _(—) end(i)_(≧t)client.lookup(i)session.minFlowRate)

It is noteworthy that the worst-case client flows at time t are expressterms of the present minimum flow rates, which cannot increase overtime, but might hold steady. Finally, a predicate is defined that testswhether a prospective customer will cause swing capacity to be exceededat some time t, as follows:boolean client_fits(i,t){if(client.lookup(i).contentSelection.averagePlayRate<=swingcapacity(t)−server.cac_flowSafetyMargin)return true; else return false;}  (18)Procedure (FIG. 11)

This procedure is an adaptation of the first, which has been extended toconsider swing capacity at times in the future when capacity undergoesscheduled changes. Before accepting a client, its minimal bandwidthrequirement (which by construction of the flow modulator will neverincrease over time) is checked against projected swing capacity atpoints in time when total available capacity undergoes scheduled stepchanges, provided these times fall within the proposed content viewingperiod. A candidate is activated only if all tests succeed.

Step 1100 builds a sequence of time values (SeqT) at which step capacitychanges are scheduled to occur. The first element of this sequence ist_now, representing the present.

Beyond step 1100 the queue of waiting clients is scanned in FIFO order,yielding a candidate designated by id at each iteration.

The bandwidth requirement for client id in Step 1120 is obtained asfollows:required_bandwidth=client.lookup(id).contentSelection.averagePlayRate

The worst-case end time for content flow to id is obtained according tothe content selected, as follows:t_end=t_now+client.lookup(id).selected.playTime

Steps 1130 through 1150 are executed within an iteration for each timepoint t in SeqT falling between t_now and t_end. This iteration is endedin step 1130 if t exceeds the time window of interest, or in step 1150if the supply of scheduled capacity changes is exhausted.

For each time value, step 1140 compares required bandwidth to projectedswing capacity.

Projected swing capacity at time t is:swingCapacity(t)=server.maxFlowRate(t)−Σ_(t)_end(i)>t(client.lookup(i)session.minimumFlowRate)

Note that only active clients whose t_end times occur after t areconsidered in the sum of minimum flow rates.

The predicate expression used in step 1140 at time t is thus:(required_badwidth<=swingCapacity(t)-server.cac_flowSafetyMargin)

Step 1160 performs the same actions as step 660 in the previous CACflowchart

The first CAC process is a special case of the present one, in which theset of step change times to server.maxFlowRate is empty (i.e.server.maxFlowRate is constant), and Seq_(T) (t_now)==t_now.

In preparing Seq_(T) (t_now), one need only consider future times thatwill pass before the longest possible content is played out if startedat t_now. In order to sidestep problems associated with rollover (atmidnight, year 2000, etc.), time is best expressed as a monotonicallyincreasing value (e.g. seconds since Jan. 1 1990).

CAC for Maximal Charges

Overview (FIG. 13)

Previously, a method for flow modulation was presented that maximizescharges to clients with active sessions. The CAC embodiment presentedpreviously was not sufficient as it does not consider the cost ofservice as a basis for connection acceptance. As a result, it may turnaway higher paying customers while granting service to lower payingones, thereby defeating the purpose for this particular embodiment.

Therefore, another embodiment is defined which offers the followingfeatures:

1. Awaiting clients are serviced in order of their respective servicecategories, higher paying clients first.

2. Once accepted, a client is guaranteed to receive acceptable serviceirrespective of its service category.

3. Under heavy load conditions higher paying customers are more likelyto be accepted than lower paying ones.

4. Lower paying customers will no be starved for service when higherpaying ones enjoy a surplus.

5. Available bandwidth is not thrown away needlessly while clients arebeing denied service.

The first objective is easily met by dividing the client queue into asmany bands as there are service categories, resulting in a banded queue.Bands are ordered within the queue according to their servicecategories, with the costliest category in front. As prospective clientsarrive and make their selection they are placed in their respectivequeue band according to their service category (which may be setcontractually, according to content selection, etc.).

Our second objective is met by employing a procedure patterned afterthose presented previously & offering the same guarantee. Toward ourthird and fourth objectives we propose dividing total availablebandwidth in as many strata as there are service categories. As depictedin FIG. 12, two service categories are shown, Premium and Basic, eachentailing an associated cost of service. A prospective client isaccepted only if there is sufficient swing capacity available within itsgiven service category. The swing capacity for a given category is givenby the smaller of 1) the difference between its maximum floor flow rate(corresponding to the top of the stratum for the service category) andthe sum of the minimum rates of all active sessions in its category orbelow, and 2) available swing capacity overall. Finally, our fifthobjective is met by allowing the flow optimizer to function freelysubject to its operational constraints. The imposed ceilings on callacceptance by category relate to minimum flow rates, which merely imposea floor on actual flow rates. For example, basic clients might wellconsume all available bandwidth 300 in the absence of any premiumcustomers, yet could be throttled back toward their floor flow rates(which together cannot exceed 200 in this example) at any time shouldany premium customer suddenly demand service. In contrast, premiumcustomers could consume the entire 300 bandwidth. As lower payingcustomers appear these would be admitted to the degree that their quotaon minimum flow is not exceeded (i.e. 200) and the availability of swingcapacity on the system.

Procedure (FIG. 13)

The present procedure requires a number of ancillary definitions, whichfollow:

Let the service categories be denoted by k=1 . . . N, where k alsodenotes the cost of service.

Let server.maxMinFlowRate[k−1] be the top of the stratum for servicecategory k. Note that server.maxMinFlowRate [N−1]=server.maxFlowRate.

Let S_(k) be the set of active client indices with a service categoryequal to or less than k. Note that S1 is contained in S2, S2 iscontained in S3, and so forth, and that S_(N)=S_(activeClients).

Let swingcapacity(k) denote available swing capacity for servicecategory k. By construction: swingCapacity(k)=minim of:(server.maxMinFlowRate[k−1]− Σ_(iεSk)client.lookup(i).session.minFlowRate))(server.maxFlowRate−Σ_(iεSactiveClients) (client.lookup(i).session.minFlowRate)))

Referring to FIG. 13, this method is used for CAC when multiple ratetariffs are in effect, and there is a desire to maximize economicreturns to the service provider while offering acceptable service toall.

All waiting clients are scanned in FIFO sequence. The actions taken inSteps 1320 and 1360 are identical to those described in connection withearlier CAC flowcharts.

Step 1340 evaluates a predicate expression that tests whether therequired bandwidth can be accommodated without exceeding the lesserof 1) swing capacity available to the client's category of service, and2) total available swing across all categories of service. The latterfactor could be determinative if all available bandwidth were allocatedto high paying customers, leaving lower paying ones such as the proposedclient unable to draw from their unfilled quota.

Let us suppose that candidate client id belongs to rate category k.

We define the swing capacity available in rate category k as:swingCapacity(k)=least of: (server.maxMinFlowRate[K−1]−Σ_(i in)_(sk)client.lookup(i).session.minimumFlowRate)) and(server.maxFlowRate−Σ_(iεSactiveClients) (client.lookup(i).session.minimumFlowRate))

The predicate expression invoked by step 1340 can now be written asfollows:(required_bandwidth<=swingCapacity(k)-server.cac_flowSafetyMargin)

This algorithm processes queued clients in band sequence, and withinevery band in FIFO order. If the predicate evaluates to true, the clientis activated. Otherwise two possible cases are considered: 1) under theFirstFit embodiment, the procedure continues scanning clients to the endof the banded queue, activating clients whose requirements can be met;2) under the FIFO embodiment, the procedure ends with the firstcandidate client whose requirements cannot be met. Many other variationson these two embodiments might also be considered.

Anticipating Scheduled Variations in Network Capacity (Maximal Charge)Overview

The procedure applicable to optimization of delivery charges is obtainedby blending elements of the CAC method depicted in FIG. 13 into themethod depicted in FIG. 11, which applies without change. To understandhow this might work it may be useful to visualize a version of FIG. 10stratified along its length in the manner of FIG. 8. As the maximum flowlevel undergoes a step change, so too do the widths of its constituentstrata in equal proportion.

Procedure

As previously mentioned, the CAC method (FIG. 11) applies to thiscircumstance also, provided we alter the definition of two routines,(17) and (18), upon which that procedure relies, yielding (20) and (21),and adopt the banded queue organization outlined in the previoussection.

The server swing capacity at a future time t is computed according tothe capacity and worst-case client flows at time t. (20)swingCapacity(k, t) = minimum of ( (server.maxFlowRate(t) *(server.maxFlowRate[k−1]/ server.maxMinFlowRate[N− 1])Σ_(i e Sk & (t)_end(i) > t) (client.lookup(i)session.mminFlowRate) ),(server.maxFlowRate − Σ_(i ε SactiveClients & (t)_end(i) > t)(client.lookup(i)session.minFlowRate)))

Finally, we define a predicate that tests whether a prospective customerwill cause swing capacity to be exceeded at some time t, as follows:(21) boolean client_fits(i, t) {  k = client.lookup(i).cosOfService; if(client.lookup(i).contentSelection.averagePlayRate <=   swingCapacity(k,t) − server.cac_flowSafetyMargin)   return true; else return flase; }

Referring to FIG. 14, a block diagram illustrating a preferredembodiment for a system implementing the methods presented herein isshown. Block 1485 depicts plurality of network-based client computersthat receive data from a server over the course of a session. Everyclient sessions has an associated session block 1435 within the serverby means which a flow rate of content from client to server is effectedand regulated to a given flow rate set-point. The flow optimizer 1430manages bandwidth utilization across all sessions that have beenadmitted by the system call admission control (CAC) and the bandwidthcall admission control blocks, labeled 1400 and 1420 respectively.Specifically, the flow optimizer 1430 modulates the flow rate set-pointof every active session so as to optimize aggregate flow or, moregenerally, cost.

Call Admission Control

Whenever a client contacts a server requesting service, the server mustdetermine whether or not to admit the client. This admission function isperformed in two separate phases. The system call admission controlblock 1400 considers whether or not sufficient internal resources existto support the request. Toward this end, the system CAC 1400 estimatesthe needs of the prospective client with respect to a set of limitedsystem resources such as I/O bandwidth, cache memory, open files,threads, etc. A request is denied if any required resource cannot beobtained. In the contrary case, the request is passed on to theBandwidth CAC, which tests if the client fits and admits or rejects theclient accordingly. The swing capacity is computed by the flow optimizer1430 on a periodic basis according to equations (14) and (17) asappropriate. The managed bandwidth must reflect the true capacity of theattached network and the adapters leading thereto. This parameter shouldnot be set arbitrarily high to prevent rejection of client requests.Doing so will cause the system to admit calls that cannot be properlyhandled, thereby diminishing the quality of the viewing experience.

Having admitted a client, block 1430 creates a client session block1435, which then begins to converse with its client-residentcounterpart, 1480) during their launch phase. Thereafter, the flowoptimizer 1430 assumes control, and modulates flow over the sessionuntil either all content is delivered, or a failure to deliver isdetected, or one of a number of VCR-like stream control events intervene(e.g. pause, rewind, fast-forward).

Server and Client Session Control Blocks

Data Transport Channel 1450 is responsible for transporting data packetsoriginating from the flow regulator 1440 within the server to theformer's peer entity 1460 within the client. The mode of communicationbetween the peer entities may be connection-oriented (e.g. TCP, whichdelivers a byte stream in order, without duplication or loss) ordatagram-oriented (e.g. UDP). Best-efforts datagram service is typicallyenhanced with acknowledgements to facilitate rapid recovery fromoccasional packet loss, out-of-sequence delivery, and duplication.

Peered session control channels 1445 and 1465 permit configuration andstatus data, together with miscellaneous commands, to be sent fromclient to server and vice-versa. A reliable connection-orientedtransport service is typically employed for this purpose (e.g. TCP).Data and commands alike are multiplexed over this channel in the serviceof other entities within the clients or server. Thus, adatagram-oriented data transport channel 1460 within a client 1485 mightemploy the session control channel to ferry selective acknowledgementsto its peer in the server. Similarly, the flow meter 1465 within theclient 1485 forwards the measured flow rate to the associated flowregulator 1440 within the server.

The flow regulator 1440 obtains content data from a cache or disk, atthe current offset into the content file, which it forwards as packetsto the data transfer channel 1450. The size and/or pacing of the packetsare determined by the flow regulator 1440 in order to achieve the flowrate set-point imposed by the flow optimizer 1430. The flow regulator1440 is also responsible for determining the channel flow capacity tothe client, which is the least of a configured value obtained from theclient and a measured value. Toward this end, during the launch phase,the flow regulator 1440 might send content data packets to the clientflow meter 1475 in a series of packet trains at progressively higherpeak flow rates ending with the configured maximum flow rate, if any.The highest measured flow rate reported by the client flow meter 1475 isaccepted as the flow rate capacity, which is posted for use by the flowoptimizer 1430. Subsequently, the flow regulator 1440 will compare theflow rate set-point with the flow rates reported by the flow meter 1465,and down grade capacity as needed should the measured flow rate fallconsistently fall below the requested flow rate. Whenever such adowngrading occurs, the flow regulator 1440 will test for subsequentrelaxation of a transient constriction by means of the aforementionedseries of packet trains in which the peak packet flow rate exceeds thedelivered flow rate.

The buffer manager 1470 accepts data from the flow meter 1465, whichdata is retained until it is consumed or passed over the contentconsumer 1480. The buffer will typically be implemented as a ring bufferin memory or on disk. The write cursor will lag the read cursor by a noless than a specifiable time interval so as to permit rewinds ofacceptable scope without requiring selective data re-retrieval from theserver.

Flow Optimizer 1430

The flow optimizer 1430 assumes control over a session once it has beenlaunched, and its channel capacity has been determined. Subsequently, itmodulates the flow rate set-point of every session such as to optimizeaggregate flow or charges across all active sessions, subject to avariety of min/max constraints on sessions flows, and a maximumconstraint on aggregate session flow, as previously discussed.

The flow optimizer 1430 continues to modulate session flow until allcontent has been delivered, or one of a number of session anomaliesintervene, as follows:

A downgraded session flow capacity drops below the minimum session flowrate. The session is dropped by the optimizer 1430 as a solutioninvolving this channel does not exist. The user is forced to apply forre-admission, with the option of accepting slower than real-timedelivery at the present encoding rate, or accepting a degraded viewingexperience at a reduced encoding rate better adapted to the presentlyavailable bandwidth.

A pause VCR command is received from the client. Flow to the client cancontinue until the client-side buffer is topped off, after which timeflow drops to zero. The session is permanently dropped should the pausepersists too long. Until that time, the CAC safety margin is temporarilyincreased by the jit (Just-In-Time) flow rate of the paused session. Inthis way the session can be resumed without the need for re-admissionand the jit bandwidth is made available to currently sessions so long asthe pause persists.

A VCR Jump-Forward command is received from the client. This action isproblematic as it causes the just-in-time flow rate (calculated at thenew forward offset) to increase, in violation of the assumptions of theCAC algorithm. It must be noted that this violation will occur even ifthe content that is sought has already been delivered to the client-sidebuffer. One workable policy is to force the client to apply forre-admission at the forward offset and with a suitably modified bufferlevel.

A VCR Jump-Backward (Rewind) command is received. This action may entailthe need for re-transmission of content, as previously discussed. Thejust-in-time flow rate will decrease as per the new position andadjusted buffer contents.

Tunable Minimum Constraint

A parameterized family of minimum flow constraints can be defined asfollows:wherein: f _(k)(t)=Target content flow rate for session k;   (22)

-   -   f_(k) ^(JIT)(t)=Just-in-time flow rate for session k;    -   (RHS of Equation (5))    -   β=Burst tuning factor (positive or zero); and    -   f_(k) ^(min)(t)=Minimum (reserved) flow rate for session k.

This family converges on f_(k)(t)>=f_(k) ^(JIT)(t) when β approaches 0.A β value of 0.1, for example, ensures a minimum flow rate that exceedsthe JIT flow rate by 10%. Thus, non-zero values of β force earlydelivery of content to a player's media buffer, with beneficial effectson the viewing experience.

Alternative Call Admission Control

The traditional CAC algorithm compares the flow rate requirement of aprospective client with the unused bandwidth (obtained by subtractingthe aggregate flow target from the server flow capacity) and grantsservice if unused bandwidth is not exceeded. This procedure may breakdown as the optimizer causes all available bandwidth to be used if itcan, thereby reducing unused bandwidth to zero in the best case, evenwhen a new client could be accommodated with a healthy margin to spare.

Nevertheless, a simple variation on this idea can be used. Availablebandwidths can be defined as the difference between server flow capacityand the sum of minimum flow rates for each session computed according to(22) as follows:f _(available)=f_(Svr) ^(max)−(f ₁ ^(max) + . . . +f _(n) ^(min)   (23)wherein: f_(available)=Available bandwidth

f_(Svr) ^(max)=Server aggregate flow capacity; and

f_(n) ^(min)=Minimum (reserved) flow rate for session n.

Service is granted if the computed minimum flow rate of the prospectiveclient is less than available bandwidth even when unused bandwidth iszero.

Toward a Tunable QOS

In the preceding discussion it is assumed that all clients share thesame value of .beta. One might also consider a scheme whereby a clientreceives a β value according to the class/cost of service that clienthas signed up for: the greater the cost (and expected quality ofservice, or QOS) the greater the value of β.

Instead of maximizing charges by the methods described above, avariation on the algorithm to maximize flow is considered whereby thereserve flow computation for every session takes into account theindividual β value of the associated client. In this fashion a singlealgorithm can serve either end. The optimizer first ensures that everysession receives its required minimum bandwidth (which would nowtypically depend on β and elapsed session time) before allocating anyexcess bandwidth more or less evenly to all by interpolation. (Accordingto FIG. 15.)

The scheme is simpler computationally than the iterative interpolationmethod shown in FIG. 16; it is also arguably more fair and stable withrespect tot he allocation of available bandwidth.

Deadline-Driven Content Distribution

In delivering viewable or audible content to a player, the optimizationalgorithm imposes a minimum flow rate that ensures that all content isdelivered by the implied deadline, which is the last moment of a playsession. Notwithstanding, the optimizer attempts to deliver contentearlier according to bandwidth availability within the server taken as awhole. A similar, though far simpler scenario arises relative to thedistribution of content from an optimized server to a target device,whenever the delivery must be accomplished by a designated time, orequivalently, within a given elapsed time, and when overlappednon-blocking consumption of the content at a fixed rate is not required.

One example concerns the application where a session bundle comprises asession associated with the featured program together with sessions forevery advertisement scheduled to interrupt the feature program at adesignated time. Each session is endowed with a separate media buffer,which, in the case of ads, would typically be large enough toaccommodate the ad in full, thereby permitting a seamless transition tothe ad at the appointed time.

In adapting the optimizer to the needs of content distribution we mustrevisit the formulation of the flow constraints, as follows:

1. Media buffers are not of a concern, let alone buffer overflow, and itcan be assumed that all content received is saved away in a file ondisk, or in memory awaiting further use.

2. For these same reasons, the value of β may be zero, at which theminimum flow rate equals the just-in-time rate obtained by dividing thecontent as yet undelivered by the time till deadline:f_(k) ^(JIT)(t_(k))−L_(k) ^(TOGO)/(T_(k)−t_(k))   (24)

wherein: L_(k) ^(TOGO)=Content to go for session k, i.e., as yetundelivered by the server at time t, (client.lookup(k).session.payloadToGo).

Given the arbitrary format of the file to be distributed, T no longersignifies the “duration of play” at the average play rate(averagePlayRate); rather, T represents the elapsed time to the deadlinemeasured at the moment the delivery session was launched, and trepresents the elapsed time since session launch.

Session Bundles

A session bundle is a set of sessions that flow along a shared channel,and are thus subject to an aggregate flow constraint equal to thecapacity of the shared channel. One important application of sessionbundles is connected with ad insertion.

It is not enough to treat each of the constituent sessions asindependent with respect to the flow optimizer, for this practice mightwell result in session flow targets that do not exceed channel capacitywhen considered singly, yet exceed this same shared capacity when summedtogether.

A solution to this problem involves two distinct steps:

1. The sessions bundle is treated as a virtual session by the serveroptimizer, subject to the minimum flow constraint obtained by summationof the individual session minima, and a maximum constraint that is theleast of (a) the sum of the least of constraints (3) and (4) for eachsession, and (b) the shared channel capacityclient.lookup(k).channel.maxFlowRate (f_(k) ^(cap)). The flow optimizerthen generates an aggregate flow target for all channels virtualsessions by interpolation, using:Cost(t)=f₁(t)+ . . . +f_(n)(t)   (25)

The cost vector in this instance is the n-dimensional unit vector, wheren is the number of active clients.c=1, 1, . . . , 1)

Now it is possible to have an achievable aggregate flow target for thesession bundle k that we must apportion optimally among the constituentsessions. Fortunately for us, our interpolation procedure of FIG. 15 canbe applied again to the sessions in our bundle, but where the aggregateflow target generated above replaces the aggregate flow constraintf_(Svr) ^(max)

Specifically:

For every flow f_(ki), within session bundle k, the optimal flow rate isobtained by interpolation between minimum and maximum session flows:f _(ki) ^(opt) =f _(ki) ^(min)+α*(f _(ki) ^(max) −f _(ki) ^(min))   (27)α=[(f _(K) ^(bundle)−f_(k1) ^(min) + . . . f _(kn) ^(min))]/[(f _(k1)^(max) −f _(k2) ^(min))++(F _(kn) ^(max) −f _(kn) ^(min))]  (28)and the value of f_(k) ^(bundle) is obtained from a higher leveloptimization in which session bundle k is treated as a virtual sessionfor which:f _(k) ^(min) =f _(k1) ^(min) + . . . +f _(kn) ^(min)   (29)f _(k) ^(max)=min(f _(k1) ^(max) + . . . +f _(kn) ^(max) , f _(k)^(cap))   (30)

It must be noted that session bundles are typically dynamic in nature,outliving many if not all of their constituent sessions. Before asession is added to the bundle two CAC decisions must be reached:

(1) establish bandwidth availability within the bundle; and

(2) bandwidth availability within the server.

Relationship to Adaptive Rate Control

The flow optimization methods should be viewed as constituting thelowest and perhaps only the first of many supervisory control layers tosit astride all streaming sessions, which comprise their ownhierarchical stack of control functions; rate adaptive control sittingabove a flow rate regulator which relies on a transport entity.

Channel capacity is known to vary widely over time. A capacity drop tobelow the minimum reserve flow rate is problematic as it causes thelatter to rise in violation of its monotonicity assumption. In a serverwith available bandwidth to spare such a rise can be accommodated by theflow optimizer (by bumping the session's minimum flow reservation to thehigher value) provided the flow deficit can be made up in the near termby draining pre-delivered content from the media buffer. Should eitherof these resources approach depletion, stream degradation to a lesserbit-rate or frame rate may be the only viable option short of a pause.Whether up or down, any rate change is a change to fe, which must bepreceded by change to session's reserve flow rate allocation by theoptimizer.

As can be seen, the optimizer and rate adapter must be integrated withcare. One possible approach would be to modify a rate adapter such thatit renegotiates its proposed fe value with the flow optimizer prior toeffecting the rate change it deems appropriate based on informationprovided by the flow regulator. Rate increases might well be subject todeferral under heavy load conditions, awaiting bandwidth availability.The opposite is true of rate flow decreases: these would be forestalledas long as possible by the optimizer on a system that is not fullyloaded, as previously discussed.

Live Events

Referring to FIG. 17, the method of the present application can beextended to the optimization of “live”, time-shifted “near-live” events,and cascaded delivery, wherein one server delivers content to anotherwhile the latter accepts clients for the same content. The extensionassumes the following characteristics of the server and the liveperformance:

1. The performance is of a fixed duration T known in advance, encoded atan average rate fe, with a projected content payload of fe*T;

2. The origin server at the point of injection delays the stream by asmall dwell time T^(DWELL) on the order of seconds (e.g. 10).Notwithstanding, the delayed content is available for early delivery tomirrored servers and players with the aim of providing these with amodicum of isolation from network jitter affecting the timely deliveryof the live video frames.

3. Throughout the live event, captured content is stored in a file,thereby permitting late joiners to view the “live event” starting fromany point between the present and the event start time;

4. At the conclusion of the live event, the encoded content, now fullycaptured to a file, is available for viewing over the balance of theadmission window (e.g., 60 minutes in FIG. 17).

The optimization of live events and video on demand (VOD) differprincipally in one respect, namely in the formulation of the servercontent underflow constraint.

For VOD, this constraint is

(31) L_(k) ^(TOGO)(t) is initialized to the content size (fe*T) at thatstart of play, and decremented as content is delivered to the client. Δtis the time interval between successive periodic optimizations (e.g. 1second). In practice this constraint rides well above other operativemaximum flow constraints (e.g. buffer overflow, channel capacity) untilthe very last calculation of the session flow target by the optimizer.

For a live stream, this constraint must additionally reflect the factthat content is streaming into the server (where it builds up), even asthe server streams it out toward clients, and that the latter activitycannot possibly overtake the former.

Referring to FIG. 17, (which illustrates how optimized live streamsbehave) line L1 depicts the build-up of freshly captured content to theserver. Line L2, which is delayed (time-shifted) relative to L1 by thedwell time T^(DWELL), represents the live performance as cast andviewable by clients. One such client is admitted at time A and joins thelive session at the live moment B. The dashed curved line linking pointsB and C shows a plausible build-up of content in the client's mediabuffer in advance of play, as the viewer continues its advance alongline L2. Past point C the top of the media buffer tracks the livecapture stream and thenceforth advances along line L1. In this instance,the time shift between the live performance as cast (L2) and the liveperformance as captured (L1) imposes a maximum on the amount of contentthat could ever accumulate in a client media buffer, irrespective of itsactual capacity.

Another client, admitted at time t=10, is interested in viewing the livestream from the beginning without missing anything. As before, thedashed line joining 10 and D depicts a plausible build-up of content inthe client's media buffer in advance of play, as the viewer continuesits advance along line L3, which is time-shifted by (10+T^(DWELL))minutes relative to L1. The greater time shift (now between L1 and L3)permits more extensive pre-buffering of “near live” content.

The present constraints do not impose suitable flow limits at and beyondpoints C and D when the client delivery streams finally links up withthe live capture stream, in the live moment. Consequently the optimizermay propose flow targets that cannot be realized.

Accordingly, the following modification for use in connection with liveor cascaded streams is made:f _(k)(t)<=[L _(k) ^(TOGO)(t)−Live^(TOGO)(t)]/Δt   (32)for a live event delivered at flow rate fe, we haveLive^(TOGO)(t)=max(fe*(T−T ^(DWELL) −t), 0)   (33)

Live^(TOGO)(t) is the span between projected content size fe*T (levelline L0 in FIG. 17) and the content capture level line L1, whereas L_(k)^(TOGO)(t) is the gap between L0 (i.e., fe*T) and the dashed curved linerepresenting the content level delivered to the media buffer. The lattermust always exceed the former. As the two approach one another thisconstraint takes effect, causing L_(k) ^(TOGO)(t) to always exceedLive^(TOGO)(t) as it decreases at ratefe.

Viewers joining late and willing to see the live performance with somedelay relative to the live case enjoy higher potential levels ofisolation from network disturbances, and thus QOS, by virtue of theirgreater effective buffer capacity. By the same token, such viewersafford the optimizer greater latitude to optimize the flow of VOD and“live” content across load fluctuations, thereby enhancing servercapacity. Viewers that wish to experience the live event in “real-time”nevertheless benefit by a modestly enhanced QOS conferred by a dwelltime's worth of content in their buffers.

SUMMARY

A method and system for call/connection acceptance and flow modulationfor network delivery of video/audio programming is thus provided.Although several embodiments have been illustrated and described, itwill be apparent to those skilled in the art that various changes andmodifications may be made without departing from the spirit of theinvention or the scope of the claims.

1. A method of bandwidth allocation for delivery of stored digitalcontent from at least one server device to at least one client device byway of a network, the method comprising the steps of: describing acontrol variable which represents a target flow rate of content from theserver device to each client device; determining time-varyingconstraints on the target flow rate of the content; determining a costfunction of the control variables for all clients, wherein the costfunction represents an aggregate flow rate and is a sum of all flowrates for all clients; and prescribing bandwidth to all clients basedupon a value of the control variables that maximize the cost functioncomprising performing periodic computations to update the value of thecontrol variable such that the bandwidth can be continuously allocatedto each client; and wherein a new client is accepted by: determining anadmission capacity of the bandwidth; admitting a prospective client ifthe clients minimum allowed value of the control variable is less thanthe admission capacity; and wherein a client admitted for service isguaranteed to have sufficient content flow over the entire session. 2.The method of claim 1, wherein the admission capacity equals a serverswing capacity reduced by a predetermined safety margin which may bezero, the swing capacity equaling a difference between a server flowcapacity and a sum of a minimum allowed flow rate for all clients. 3.The method of claim 2, wherein the minimum allowed value of the controlvariable is initialized to an average consumption rate.
 4. The method ofclaim 2, wherein the minimum allowed value of the control variable isinitialized to the average consumption rate multiplied by a factorgreater than one.
 5. The method of claim 2, wherein the server flowcapacity varies in a step-wise function over time according to apredetermined schedule and the method of accepting a new client furthercomprises: determining a sequence of future step changes of server flowcapacity; determining at each time in the sequence of future stepchanges the value for a future worst swing case capacity, the worst caseswing capacity being obtained from the server flow capacity at the timeby subtracting an extrapolated present minimum allowed flow rate for allactive clients that are potentially active in the future; admitting aprospective client if a minimum allowed value of the control variable isless than a capacity obtained from the sequence and a present capacity;and whereby a new client can be admitted without compromising theservice to existing clients in the presence of previously scheduledserver flow capacity changes and all clients accumulate content in theirbuffers to the extent of their respective buffer capacity.
 6. The methodaccording to claim 5, wherein the method of accepting a new clientfurther comprises: partitioning the available bandwidth into partitionsrepresenting a prescribed service charge; determining a maximum-minimumcapacity for a service charge to be the maximum allowed sum of allminimum allowed flow rates for all clients incurring an equal or lesserservice charge, the maximum-minimum capacity equaling the sum ofbandwidth allocations for all service charges less than or equal to theservice charge; determining a service-charge-specific swing capacity fora given service charge as the difference between the maximum-minimumcapacity for the service charge and the sum of all minimum allowed flowrates for all clients incurring the service charge or a lesser servicecharge; determining the server swing capacity to be the server flowcapacity minus a margin and minus the sum of the minimum allowed flowrate for all clients; and determining the service-charge-specificcapacity to be the least of the server swing capacity and the chargespecific swing capacity.
 7. The method of claim 6, wherein the method ofaccepting a new client further comprises: determining the sequence offuture times of step changes to server flow capacity over a proposedconsumption period; determining for each time in the sequence of futuretimes, the future worst-case value for the service-charge-specificcapacity, the worst case value being obtained by using the extrapolatedpresent minimum allowed flow rate of all presently active clients thatare potentially active at the time in the future; and admitting a clientincurring a given service charge if the minimum flow rate of the clientis less than the capacity obtained from the sequence and the presentservice-charge-specific capacity.
 8. The method of claim 3, wherein thestep of determining time-varying constraints comprises determiningtime-varying constraints wherein: an aggregate flow rate for all clientsdoes not exceed a predetermined server flow capacity; a flow rate fromthe server to the client does not exceed a maximum allowed flow rate forthe client; a flow rate to the client will never cause a buffer of theclient to overflow; a flow rate to the client stops when the content isexhausted; and a flow rate from the server will never be less than aminimum allowed flow rate, which may vary over time and may be zero. 9.The method of claim 8, wherein the maximum allowed flow rate to theclient is given by the minimum of one selected from the following: aclient flow rate ceiling; a flow rate required to fill the client bufferbefore the next periodic computation; and a flow rate required tocomplete the delivery of the content before the next periodiccomputation.
 10. The method of claim 9, wherein: the minimum allowedclient flow rate cannot increase over time, but may decrease such thatthe initial value of the control rate is no less than the averageconsumption rate; the control variable for the client is always greaterthan or equal to the current minimum allowed client flow rate; and theclient buffer will never underflow.
 11. The method of claim 10, whereinthe minimum client flow rate is the product of the constant content flowrate from the client to the server that causes the last piece of contentto be delivered at the last possible moment as the client buffer isbeing drained at the average consumption rate, with a factor greater orequal to one.
 12. The method of claim 9, wherein the minimum client flowrate is initialized to the average consumption rate.
 13. The method ofclaim 9, wherein the minimum client flow rate is initialized to theaverage consumption rate multiplied by a factor greater than one. 14.The method of claim 9, further comprising the step of calculating theminimum client flow rate periodically.
 15. The method of claim 9,further comprising the step of calculating the maximum client flow rateperiodically.
 16. The method of claim 1 wherein the cost functionrepresents an aggregate flow rate and is the sum of the flow rates forall clients.
 17. The method of claim 1 wherein the cost functionrepresents a service charge and is the sum of the client flow ratesmultiplied by the client's cost of service.
 18. The method of claim 17,wherein: the step of describing the control variable comprises:determining a maximum allowed flow rate and a minimum allowed flow ratefor each client; determining a flow rate range for each client as thedifference between the maximum allowed flow rate and the minimum allowedflow rate; and initializing a current flow rate for each client as theminimum allowed flow rate and summing the flow rate into the totalserver flow rate; and the step of prescribing bandwidth comprises:computing remaining server bandwidth as the difference between themaximum server flow capacity and the total server flow rate; andallocating remaining server bandwidth to remaining clients wherein allclients with a given cost of service receive bandwidth equally to theextent of their respective ranges, and no client with a given cost ofservice being allocated bandwidth unless all clients with a higher costof service have received their maximum possible allocation of bandwidth.19. The method of claim 15, wherein the step of describing the controlvariable comprises: computing the sum of the minimum allowed flow ratesto all active clients; computing the sum of the flow rate ranges for allactive clients; computing the difference between the maximum allowedaggregate flow rate and the sum of the minimum; computing a factor asthe ratio of the difference over the sum of the ranges; if the factorexceeds 1, the control variable is set to the maximum allowed clientflow rate; and if the factor is less than 1, computing the controlvariable by multiplying the client range by the factor and then addingthe result to the minimum allowed client flow rate.
 20. The method ofclaim 15, wherein: the step of describing the control variablecomprises: determining the maximum allowed flow rate and minimum allowedflow rate for each client; determining the flow rate range for eachclient as the difference between the maximum flow rate and the minimumflow rate; and initializing the control variable for each client as saidminimum allowed flow rate and summing the flow rate into a total serverflow rate; and the step of describing the control variable comprises:computing remaining server bandwidth as the difference between themaximum server flow capacity and the total server flow rate; andallocating remaining server bandwidth to remaining clients to the extentof their respective ranges.
 21. The method of claim 19, wherein the stepof allocating remaining server bandwidth further comprises the clientsreceiving the bandwidth equally to the extent of their respectiveranges.
 22. The method of claim 17, wherein the maximized value of thecontrol variable can be determined by: determining a maximum and minimumflow rate for each client; determining a range between the maximum andminimum flow rates in order to find a flow rate range; determiningavailable bandwidth by finding the difference between an aggregate flowcapacity and a sum of the minimum flow rate; determining a flow variableby dividing the unused bandwidth by the flow rate range; and prescribingthe control variable to be the minimum flow rate added to the flow raterange if a flow variable is less than one or prescribing the controlvariable to be the minimum flow rate added to the flow rate rangecorrected by a flow variable if the flow variable is greater than onesuch that the control variable is easily calculated for each client. 23.The method of claim 15, wherein at least one session is a session bundleand: the minimum flow constraint is computed as the sum of the minimumflow rate constraints for all bundled sessions; the maximum flowconstraint is the least of the sum of the maximum individual flow rateconstraints of each session and the flow capacity of the channel sharedamong the bundled sessions; and the target flow rate is computed for thesession is apportioned among the bundled constituent sessions.
 24. Themethod of claim 23 wherein the bundled session is an advertisement. 25.The method of claim 1, wherein the steps of prescribing a controlvariable, and determining a cost function of the control variables forall clients are performed based upon an amount of buffer of the client.26. The method of claim 8, wherein, for one or more clients, the clientflow rate ceiling is set equal to the minimum allowed flow rate, causinga bounding [min,max] segment, which constrains the flow target of anyaffected client, to shrink to a single point.
 27. The method of claim 8,wherein the minimum allowed client flow rate descends in a series of oneor more steps, starting from an initial flow rate that exceeds theaverage consumption rate.
 28. The method of claim 8, wherein for one ormore clients, flow rate ceiling is set equal to the minimum allowedclient flow rate, causing a bounding [min,max] segment, which constrainsthe flow target of any affected client, to shrink to a single point. 29.The method of claim 8, wherein target flow rates are recomputed whenevera new client is admitted.
 30. The method of claim 8, wherein the targetflow rates are recomputed whenever a minimum allowed client flow rateundergoes a step change.
 31. The method of claim 8, wherein the minimumallowed client flow rate may step up over time to the extent ofavailable admission capacity, such that no existing sessions arecompromised.
 32. The method of claim 22, wherein, for one or moreclients, the client flow rate ceiling is set equal to the minimumallowed flow rate, causing a bounding [min,max] segment, whichconstrains the flow target of any affected client, to shrink to a singlepoint.
 33. A method of bandwidth allocation for delivery of digitalcontent from at least one server device to at least one client device byway of a network, the method comprising the steps of: storing contentfrom a live event into the at least one server device, the live eventhaving a fixed duration; injecting a delay into the delivery of contentfrom the at least one server device to the at least one client device;and delivering content from the at least one server device to the atleast one client device, wherein bandwidth is allocated to the at leastone client as follows: describing a control variable which represents atarget flow rate of content from the server device to each clientdevice; determining time-varying constraints on the target flow rate ofthe content; determining a cost function of the control variables forall clients, wherein the cost function represents an aggregate flow rateand is a sum of all flow rates for all clients; and prescribingbandwidth to all clients based upon a value of the control variablesthat maximize the cost function comprising performing periodiccomputations to update the value of the control variable such that thebandwidth can be continuously allocated to each client.
 34. The methodof claim 33, wherein a new client is accepted by: determining anadmission capacity of the bandwidth; admitting a prospective client ifthe clients minimum allowed value of the control variable is less thanthe admission capacity; and wherein a client admitted for service isguaranteed to have sufficient content flow over the entire session. 35.The method of claim 33, wherein upon conclusion of the live event,storing the content from the live event for the balance of the admissionwindow.
 36. The method of claim 34, further comprising permitting newclients to view the live event starting from any point between thepresent and the event start time.